(1) Field of the Invention
The present invention relates to a photomask used in the production of a density integrated circuit such as an LSI, blanks for the photomask for the production of the photomask and a method for forming a pattern by using the halftone phase shifting photomask, and in particular the invention relates to a halftone phase shifting photomask by which a projected image having fine dimensions can be obtained, blanks for the halftone phase shifting photomask for the production of the halftone phase shifting photomask and a method for forming the pattern by using the halftone phase shifting photomask.
(2) Description of the Prior Art
Semiconductor integrated circuits such as an LST are produced by repeating the so-called lithographic process in which-a photomask is used. Particularly, for the formation of products having fine dimensions, the use of a phase shifting photomask such as that described for example in Japanese Patent Laid-Open No. 173744 of 1983 (Showa 58), Japanese Patent Publication No. 59296 of 1987 (Showa 62) and others are studied. Especially, the so-called halftone phase shifting photomask such as that described for example in U.S. Pat. No. 4,890,309 attracted the attention of people from a viewpoint of early practical application. Some proposals are made as to the improvement in yield and the structure and material that result in a decrease in cost which proposals are described in Japanese Patent Laid-Open No. 2259 of 1993 (Heisei 5), Japanese Patent Laid-Open No. 127361 of 1993 (Heisei 5) and others, and making such proposals practicable is proceeding.
A halftone phase shifting photomask is explained briefly referring to figures. FIG. 1 comprises explanatory views showing the principle of halftone phase shifting lithography. FIG. 2 comprises explanatory views showing a conventional method. FIG. 1(a) and FIG. 2(a) are each a sectional view of a photomask, respectively. FIG. 1(b) and FIG. 2(b) show the amplitude of light on the photomask, respectively. FIG. 1(c) and FIG. 2(c) show the amplitude of light on a wafer, respectively. FIG. 1(d) and FIG. 2(d) show the strength of light on the wafer, respectively. Numerals 11 and 21 (FIG. 1(a)) designate a transparent substrate. Numeral 22 FIG. 2(a)) designates 100% lightproof film. Numeral 12 designates a halftone phase shifting film. Numerals 13 and 23 designate incident rays. Halftone phase shifting film is a film having the function that the phase of applied rays which have passed through the film is inverted in fact against the phase of applied rays passing through air with the same length of light paths and the optical strength of the former is attenuated against the optical strength of the latter, which is formed of a layer or multiple layers. In a conventional method, as shown in FIG. 2(a), 100% lightproof film 22 made of chromium and others is formed on substrate 21 made of quartz glass and others, by which a light-transmitting area having a desired pattern is only formed, wherein as shown in FIG. 2(d), the distribution of optical strength shows the spread of edges so that the resolution is decreased. On the contrary, in the halftone phase shifting lithography, since the phase of rays which have transmitted through halftone phase shifting film 12 of semitransparent film is inverted in fact against phase of rays which have passed through openings of the halftone phase shifting film 12, as shown in FIG. 1(d), the optical strength in the border of the pattern formed on the halftone phase shifting photomask becomes 0 on the wafer so that the spreading of both edges is suppressed, therefore the solution being improved.
Special attention should be paid to that in order to obtain the effect of halftone phase shifting photomask, the phase angle and the transmittance of halftone phase shifting film become very important, adding to various characters such as the dimensional tolerances, the positional tolerances and others of pattern formed on the photomask which are requested to a conventional photomask, and further that the phase angle and transmittance are determined according to refractive index, coefficient of extinction in the wavelength of the applied rays and the thickness of film of a layer or multiple layers forming the halftone phase shifting film.
Generally, the most suitable value for the phase angle is 180xc2x0 . However, the most suitable value for the transmittance is in the region of 1 through 20% supposing the transmittance in the opening area of halftone phase shifting film is 100%, which is determined according to the pattern to be transferred, transferring conditions and others. As to the halftone phase shifting photomask, it is requested to set the phase angle and the transmittance to the most suitable values thereof. When values of them deviate from the most suitable value, a proper quantity of the change of applied rays appears, which results in the lowering of the dimensional tolerance, and the lowering of the focal latitude or allowable extent and others. Accordingly, refractive index, coefficient of extinction, the accuracy of the thickness of film and stability are important needless to say. FIGS. 3 and, 4 show the results of simulation made about the effect of transmittance change and phase difference change in the lithography using halftone phase shifting photomask on a change of focal depth, transferring length and best focus.
Together with a miniaturizing pattern to be formed, it is needed to shorten the wavelength of rays used in lithography. As the miniaturization over the so-called 2.5 micron design rule proceeds, the practical application of a KrF excimer laser (wavelength: 248 nm) starts, and further the length of pattern is miniaturized. In consideration of the above-mentioned conditions, the use of ArF excimer laser is studied. As to a halftone phase shifting film used in a halftone shifting photomask, the development of material is requested which enables the most suitable phase angle and transmittance to also be realized for these wavelengths and has the stable refractive index and coefficient of extinction.
Accordingly, it was proposed to use film containing mainly chromium and including fluorine as disclosed in Japanese Patent Laid-Open No. 110572 of 1995 (Heisei 7) as halftone phase shifting film. This film enables the range of values of suitable phase angle and transmittance against the above-mentioned wavelength to be realized. Further, this film has the advantage of the production of blanks, the manufacturing process of the mask and others being possible in the same manner as these of conventional photomasks. Therefore, the investigation of this film was begun from the early stage, and practical application was made.
However, the halftone phase shifting film containing mainly chromium and including fluorine has a problem that by applying rays used in the application of rays such as ArF excimer laser, over a long time, the refractive index, and coefficient of extinction, or any of the two of the halftone phase shifting film change(s). Therefore, in the case of forming a resist pattern using the halftone phase shifting photomask having the halftone phase-pattern film that contains mainly chromium and includes fluorine, the refractive index and phase angle, or any of the two, changes over time as the halftone phase shifting photomask is used.
In this case, as is evident from the result of a simulation shown in FIGS. 3 and 4, even if the phase difference and transmittance slightly change, focusing position, and latitude as well as transferring length change greatly. Namely, these changes result in the possibilities that a proper quantity of the change of applied rays, the lowering of dimensional tolerance and the lowering of focal latitude appear every time a photomask is used, and that even in one usage of photomask, these change, which results in a decrease in latitude of pattern forming and the worsening of the shape of pattern.
Accordingly, it is an object of the present invention to provide blanks for a halftone phase shifting photomask used for the production of the halftone phase shifting photomask in which transmittance change and phase angle change do not appear even if an excimer laser for expose is applied to the photomask for a long time.
It is a second object of the present invention to provide a halftone phase shifting photomask in which transmittance change and phase angle change do not appear even if the excimer laser for expose is applied to the photomask for a long time.
It is a third object of the present invention to provide a method of forming a pattern using the blanks for the halftone phase shifting photomask.
A first type of blanks for a halftone phase shifting photomask according to the present invention comprises a halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which heat treatment is given to the halftone phase shifting film, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
A second type of blanks for the halftone phase shifting photomask according to the present invention comprises a halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which a protective film is provided on the heat-treated halftone phase shifting film, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
A third type of blanks for the halftone phase shifting photomask according to the present invention comprises a halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which the surface layer of the heat-treated halftone phase shifting is removed by etching the surface, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased, and the transmittance thereof is increased.
In the first type of blanks for the halftone phase shifting photomask, it is desirable that an interface disappears from the inside of the halftone phase shifting film by the heat treatment of the halftone phase shifting film.
In the second type of blanks for the halftone phase shifting photomask, it is desirable that the protective film includes at least chromium and fluorine, and the content of fluorine contained in the protective film is lower than the content of fluorine contained in the halftone phase shifting film. Further, it is desired that the protective film is a transparent film.
A first type of halftone phase shifting photomask according to the present invention comprises a pattern of halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which heat treatment is given to the halftone phase shifting film, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
A second type of halftone phase shifting photomask according to the present invention comprises a pattern of halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which after the halftone phase shifting film has been formed into a desired pattern, the halftone phase shifting film is heat-treated, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
A third type of halftone phase shifting photomask according to the present invention comprises a pattern of halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which a protective layer is provided on the pattern of halftone phase shifting film, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
A fourth type of halftone phase shifting photomask. According to the present invention comprises a pattern of halftone phase shifting film including at least chromium and fluorine on a transparent substrate, in which after the halftone phase shifting film has been formed into a desired pattern, a protective layer is provided on the whole surface of the photomask, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
In the first type of halftone phase shifting photomask, it is desired that after heat-treating the halftone phase shifting photomask, a protective film is provided on the heat-treated halftone-shifting film, thereafter the protective film and the halftone phase shifting film are formed into a desired pattern, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
Further, in the first type of halftone phase shifting photomask, it is desired that after heat-treating the halftone phase shifting film, the heat-treated halftone phase shifting film is formed into a desired pattern, thereafter a protective film is provided on the whole substrate, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
In the second type of halftone phase shifting photomask, it is desired that after heat-treating the halftone phase shifting film formed into a desired pattern, a protective film is provided on the whole surface of the photomask, by which a change of optical characteristic of the halftone phase shifting film appearing in applying the excimer laser for exposure to the halftone phase shifting film is decreased.
In the first and second types of halftone phase shifting photomasks, it is desired that by heat-treating the halftone phase shifting film, the interface disappears in the inside of the halftone phase shifting film by heat-treating the halftone phase shifting film.
In the third and fourth types of halftone phase shifting masks, it is desired that a protective film includes at least chromium and fluorine, and the content of fluorine contained in the protective film is lower than the content of fluorine contained in the halftone phase shifting film. Further, it is desired that the protective film is transparent.
Further, in any of the above-mentioned halftone phase shifting photomasks, it is also desired that when forming a pattern, a size of the pattern is made smaller than the desired size.
Further, the present invention comprises a method of forming a pattern which includes a method of forming a pattern using the above-mentioned halftone phase shifting photomask.